1. Field of the Invention
This invention is concerned with measuring the relative and absolute velocity of water, or fluids mixed with water, through the tubing-casing annulus as applied to oil-field operations. The method employs neutron-excited oxygen activation combined with a mechanical water velocity measurement by a logging instrument deployed within the tubing.
2. Discussion of Related Art
Depleted oil production zones may be rejuvenated by water flooding. In this process the casing opposite the formation under consideration is plugged off by packers above and below the production zone. Water injection tubing, nested within the casing of an injection well, allows injection water to flow down the tubing to the desired injection zone, exit the tubing through specialized hardware, and flow in the annulus between the tubing and the casing. From this annulus the water enters the formation through perforations in the casing, pushing the oil in the formation ahead of the water-flood front. For good and sufficient reasons it is very important to the operator to monitor the velocity of the water flow in the tubing-casing annulus, and therefore the volumetric flow into a particular zone of the formation, throughout the vertical profile of the well.
Various methods are known for measuring the water-flow velocity through a single conduit such as the injection tubing itself. For example, the water flow velocity within the injection tubing can be measured by placing a spinner-type flowmeter inside the tubing. However, that method can not, of course, be used to measure the flow velocity in the annulus between the tubing and the casing which is inaccessible to a mechanical flowmeter.
Methods of measurement, which do not rely upon direct access to the flow stream in the tubing-casing annulus, have been described to measure the velocity of water flowing in the annulus. A radioactive tracer, for example, could be mixed with the injected fluid to monitor the progress of the fluid with detectors sensitive to radioactive decay. That method is unattractive first, because of the need for handling radioactive substances and second, because the results are highly qualitative.
Another well known method, referred to as oxygen activation, may be used to measure the velocity of water flow inaccessible to direct measurement. This method consists of continuously irradiating the oxygen nuclei of the flowing water with high-energy neutrons to generate therefrom the unstable isotope N.sup.16, which has a half life of 7.13 seconds. As the isotope decays, gamma rays are emitted which are counted by two radiation detectors downstream of the source that are spaced a distance, s, apart. The time behavior of the recorded count rates as seen at the two detectors follow the relation EQU C.sub.1 =K exp{-.lambda.t1} (1)
and EQU C.sub.2 =K exp{-.lambda.t2} (2)
where C.sub.1 and C.sub.2 are the respective count rates at times t1 and t2, at distances d1 and d2=(d1+s) from the source where d1 is the distance from the source to the near detector, K is the decay rate at time t1=t2=0, and .lambda. is the N.sup.16 decay constant. Using these relations and expressing time as the ratio of distance to velocity, the velocity of activated water flowing between the two detectors, V.sub.w, is given by EQU V.sub.w =s.lambda./(1n(C.sub.1)-1n(C2)) (3)
Another method for using oxygen activation to measure the velocity of water, referred to as the impulse method, differs from the above method in that the source is turned on and off, for example, in a 10-seconds-on, 60-seconds-off pattern. In this method, a localized volume of water in the region of the source is activated when the source is on and its time of passage past the detector is noted as a peak in the count rate. The velocity of the flow may be calculated from the time of passage and the known distance from the source to the detector.
While all of the current methods that employ oxygen activation to measure water flow are capable of measuring the velocity of water flow in the tubing-casing annulus, none can do so, for practical purposes, if there is an additional, physically separate stream of water present flowing in the same direction (a co-directional flow stream) at a different velocity. In that case the neutron source bombards the O.sup.16 nuclei of both volumes of water at the same time and the detectors cannot distinguish between the counts from the two flow volumes because of superposition of the decay activity. Hence, the measurement of the velocity of water flow in the presence of multiple co-directional flow streams is not possible using the current methods of oxygen activation.
Some of the applicable patents include U.S. Pat. No. 3,603,795, issued Sep. 7, 1971 to L. A. Allaud, entitled Method and Device to Measure the Speed of Water in a Polyphase Flow. This patent teaches a method substantially the same as the method described by equations (1), (2), and (3). That patent also directs its application to the detection of water flow outside the casing as well as inside.
A trilogy of U.S. Pat. No. 4,032,780, issued Jun. 28, 1977, U.S. Pat. No. 4,032,778 issued Jun. 28, 1977, and U.S. Pat. No. 4,035,640 issued Jul. 12, 1977, all to Hans J. Paap et al. teach various aspects of measuring water flow in the region outside the well casing where water might flow between stratigraphic levels through channels in the casing-cement-formation annuli. Such channels are the result of an incomplete cement seal between the casing exterior and the borehole wall in the formation. Water flowing in these channels is referred to as "behind casing water flow".
The '780 patent teaches a method to measure the volume flow rate of behind casing water flow by using a measurement of the flow velocity and an estimate of the distance R to the flow region. The velocity of the undesired water flow is calculated from the count rates in substantially the same manner as described using equations (1), (2), and (3).
The '778 patent teaches a relationship for the count rate ratio of two distinct energy regions of the gamma ray spectrum as a function of the distance from the gamma ray source. The distance to the flow channel is determined using this ratio. The calculation of the flow velocity is made in substantially the same manner as described using equations (1), (2), and (3). Using the measured velocity and distance to the channel, the volumetric flow rate may be determined.
The '640 patent teaches that background radiation due to prompt (n,.gamma.) radiation is largely avoided if the high energy neutron source is quickly pulsed and the measurements of activation count rates are made between the pulses. The linear water flow velocity of the undesired flow is calculated from the count rates in substantially the same manner as described using equations (1), (2), and (3).
In a paper entitled Applications of Oxygen Activation for Injection and Production Profiling in the Kuparuk River Field, published as paper 22130 in May, 1991 by the Society of Petroleum Engineers, H. D. Scott et al. teach use of a stationary logging instrument for measuring fluid velocity by oxygen activation using the impulse method described above. Referring to the interference from co-directional flows, the authors state that ". . . If flow does exist inside the tubing from zones below the packer, it may be difficult or impossible to quantitatively interpret the data from the zone of interest because of superposition of the flowing signals . . ." In the application under discussion in the Scott, et al. paper, the annular flow velocity to be measured is in the upward direction and the interfering flow is the tubing flow in the upward direction from zones beneath the region of the measurement. Later in the paper, the authors point out that if co-directional water flows having widely different velocities are present (such as 3 feet per minute and 100 feet per minute), the two velocities may be individually measured by use of a third, long-spaced detector for measuring the fast flow.
None of the presently available art solves the need for a general and practical method capable of measuring the velocity of the fluid flow in the annulus between the inner and outer conduit in the presence of co-directional flow in the inner conduit where two separate conduits are nested together. None of the references are directed to a general and practical method for measuring the flow velocity in the tubing-casing annulus where the difference between the flow velocities in the tubing and the annulus is not great.